This invention relates generally to methods for producing printed circuit boards and to a printed circuit board prepared by the method of this invention. In particular, it relates to a new method of forming fine circuit lines and to printed circuit boards having fine circuit lines.
In the typical production of printed circuit boards, copper foil is laminated to an insulating substrate, most often a glass reinforced epoxy resin prepreg. That laminate structure is further processed to convert the copper foil layer into a circuit pattern by selectively removing portions of the copper foil by chemical etching. FIGS. 1 and 2 illustrate cross-sections and the process steps, respectively, from a conventional pattern plating process for forming circuit lines on a printed circuit board.
Metallic foils are generally produced by an electrochemical process in a cell containing an anode, a cathode, an electrolyte solution containing metal ions, and a voltage source. When the voltage is applied between the anode and the cathode, the metal ions from the solution are deposited on the cathode to form the foil. The surface of the foil that is located, during formation, adjacent to the cathode may be referred to herein as the shiny side of the foil. The opposing side, which faces the anode and the electrolyte solution during formation of the foil, may be referred to herein as the matte side of the foil.
Thin copper foils have been applied to substrates from an aluminum supporting layer, with the aluminum being etched away to form a copper-clad substrate. A disadvantage of using aluminum as a supporting layer is that highly caustic etchants are required to etch away the aluminum. In addition, after etching the aluminum supporting layer, a desmutting step is required. It is desirable to avoid the desmutting step and the additional handling steps arising from the contamination of the etchant with dissolved aluminum.
As shown in FIG. 1, a protective resist layer is applied and cured prior to etching so that an etchant may be applied to the copper foil to create the desired circuit pattern. Ideally, the etchant would remove the unprotected copper foil in such a manner as to leave circuit lines with vertical sides.
A disadvantage of the process shown in FIGS. 1 and 2, however, is that the etchants actually do not create vertical sides of the circuit lines. Instead, the etchants tend to etch away too much copper at the top of the circuit line by undercutting the photoresist, leaving a somewhat trapezoidal-shaped circuit line. As a result, the minimum width of the circuit lines is limited by the need to allow for this non-uniform etching.
This problem was discussed in U.S. Pat. No. 5,437,914 (the "'914 patent") and it was shown that the shape of the etched circuit lines was affected by the shape of the grain structure of the copper foil. Improved accuracy of etching was to be obtained, according to the '914 patent, by treating the smooth or shiny side of the copper foil and then laminating the copper foil to the substrate with the smooth or "shiny" side down, which is contrary to the conventional practice. As shown in FIG. 1, the conventional practice involves laminating the copper foil to the substrate with the matte finish side of the copper foil being located adjacent to the substrate. An improved etching factor was obtained from the copper clad laminate of the '914 patent, indicating that the sides of the circuit lines were more nearly vertical.
Another approach to improving the accuracy of circuit lines is to use thinner copper foils, since they can be etched quickly with less undercutting. However, such foils are not easy to handle. Consequently, it has been proposed to deposit thin layers of copper on supporting sheets which can be removed after the foil has been laminated to a substrate. One example is found in U.S. Pat. No. 3,998,601 in which a 2-12 .mu.m layer of copper is deposited on a conventionally thick copper foil (say 35-70 .mu.m) and separated from the thicker foil by a release layer. After laminating the composite foil to a substrate, the supporting copper foil is mechanically stripped away, leaving the thin 2-12 .mu.m foil ready for processing into an electronic circuit. One disadvantage of this approach is that such a procedure may result in removing portions of the thin foil when the supporting foil is stripped away.
Another known alternative for producing fine-line patterns is shown in U.S. Pat. No. 5,017,271 to Whewell et al. In accordance with the technique of Whewell et al., a layer of a first metal, such as chromium or nickel, is deposited on the untreated matte side of a copper foil to produce a composite. The composite is then laminated to a support layer, with the chromium layer being sandwiched between the copper foil and the support layer. Next, all of the copper foil is removed, and a photoresist is applied to the exposed chromium layer. The photoresist is then masked, irradiated and developed to expose the chromium layer according to the desired pattern. Copper is then deposited onto the exposed chromium layer and the remaining photoresist, and underlying chromium layer, are removed rendering a finished fine-line pattern. The contents of U.S. Pat. No. 5,017,271 are incorporated herein by reference.
As shown below in the comparative examples, a disadvantage of the techniques shown by Whewell et al. is that the chromium/copper composite may not adhere sufficiently to the support layer. It is desirable for the composite on the support layer to have a peel strength in excess of 6 lb/in. A further disadvantage of the technique shown by Whewell et al. is that the deposited copper layer may not adhere sufficiently to the exposed chromium layer. It is desirable for the deposited copper layer to adhere to the chromium layer and to otherwise resist peeling from the chromium layer.
It would therefore be desirable to have an improved method for forming conductive traces and printed circuits made thereby.